local: Add support for setting watermark
local: Add support for setting watermark

Add support for setting the watermark value, if the driver supports it.
The watermark value represents the number of samples that will be read
in one burst from the hardware. Since we read data at the granularity of
iio_buffer blocks, it makes sense to use the highest watermark value up
to the buffer's size. If the value is too high, the driver will
automatically adjust it to the maximum watermark value possible.

Fixes #780.

Signed-off-by: Paul Cercueil <a027184a55211cd23e3f3094f1fdc728df5e0500@crapouillou.net>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio